Thin film growth using two part metal solvent

ABSTRACT

A method of coating a material surface with thin film silicon comprises dissolving silicon in a metal solvent to form a solution and subsequently deposited the dissolved silicon from the solution by controlling the temperature of the solution and thereby depositing a layer of silicon onto the material surface. The metal solvent is preferably a mixture of gold and a metal or metals which either have a melting point below the deposition temperature range or which form a eutectic with gold and have a eutectic temperature below the deposition temperature range. The temperature of the solution is controlled so that the silicon becomes super saturated in the solution and is deposited out of solution onto the material surface.

CROSS-REFERENCES

This application is a continuation-in-part of our earlier filed, nowabandoned, U.S. application Ser. No. 07/592,923 filed on Oct. 4, 1990which is a continuation-in-part of our earlier filed, now abandoned,U.S. application Ser. No. 07/322,758 filed Mar. 13, 1989, whichapplications are incorporated herein by reference in their entirety andto which applications we claim priority under 35 USC §120. Further, weclaim priority under 35 USC §119 with respect to earlier filedAustralian application PI 7209 filed Mar. 11, 1988 and PI 8959 filedJun. 23, 1988.

FIELD OF INVENTION

The present invention relates to a method of coating materials with thinfilms of silicon which coated materials are used in various electronicapplications, such as thin film silicon solar cells. More particularly,the invention provides a method of manufacturing such solar cells usingthe thin film coating technology.

BACKGROUND OF INVENTION

At present, the fabrication of silicon solar cells involves the use ofself-supporting crystalline or polycrystalline wafers. However, thereare a number of advantages which can be obtained by the fabrication ofsilicon solar cells in the form of thin silicon films on a supportinglayer. Accordingly, there is an increasing amount of interest withrespect to commercially applicable methods of fabricating silicon solarcells in thin-film form.

There is also an increasing interest in depositing silicon of electronicquality over large areas for consumer applications such as liquidcrystal displays for application as television screens. One suchdeposition technique is carried out by first dissolving silicon in amolten metal so that the melt is saturated with silicon and thencooling. Upon cooling, the amount of silicon which can be dissolved inthe melt decreases. The excess silicon can be made to precipitate, outonto a substrate, at a controlled rate.

At present, tin (Sn) is the most commonly used metal for dissolvingsilicon for subsequent deposition. Tin is desirable in that it iselectrically quite inert in silicon. Accordingly, the unavoidableincorporation of tin in the deposited silicon layer does not detractfrom the layer's electronic properties. However, tin is not desirable inthat high temperatures (greater than 900° C.) are required to dissolvemuch silicon in tin. The high deposition temperature severely limits thechoice of substrate material due to thermal mismatch considerations. Afurther limitation upon substrate choice as well as upon the choice ofprocessing conditions and cleanliness requirements is provided by themuch higher prospects for contamination of the silicon layer by otherimpurities at high temperatures.

Gold forms a eutectic with silicon and has a unique ability to dissolvelarge amounts of silicon at low temperatures. The eutectic compositionconsists of about 18% silicon (atomic basis) with a correspondingeutectic temperature of about 363° C. This means that at any temperaturehigher than this, a molten solution of silicon in gold can be formedwith the silicon content being at least as high as at the eutectic.Although gold is very detrimental to the electronic properties of thesilicon layer, (when incorporated into this layer in even very smallquantities) the low deposition temperature means that virtually no goldis incorporated into the lattice structure of the silicon layer.Similarly, only small quantities of other impurities will beincorporated at such low temperatures. Accordingly, processing can becarried out using relaxed cleanliness requirements for (1) the substratematerial; and (2) the deposition equipment. In addition, the purityrequirements of the substrate and the solutions employed can be reduced.The low deposition temperatures also reduce thermal mismatch problemsbetween the deposited silicon layer and the substrate.

The use of gold in silicon deposition-processing is not desirablebecause of the high solubility of silicon in gold at the eutectic. Largeamounts of silicon required to form a melt at low temperatures givingrise to difficulties in controlling the precipitation rate of siliconfrom the solution upon cooling. This reduces the crystallographicquality of the deposited film and also gives rise to the possibility ofdesirable macroscopic gold inclusions in the deposited film.

SUMMARY OF THE INVENTION

An efficient and economical method of coating a material surface with ahighly pure thin film of silicon is disclosed. The method involvesdissolving silicon in a molten metal solvent to form a solution. Thedissolving is carried out a first temperature such that the percentageof silicon in the solution is greater than the saturated percentage ofsilicon at a second temperature. The solution of silicon and metal isthen brought into contact with an area of a material surface which is tobe coated. The temperature of the silicon/metal solution is controlledwithin the range between the first temperature and the secondtemperature in such a manner so as to control the rate of deposition ofsilicon from the solution onto the material surface. By using thismethod, a highly pure silicon thin film can be deposited in a controlledmanner on the material surface. In accordance with the invention it isimportant that the silicon/metal solution be particularly comprised soas to obtain the desirable results of the invention. In order to obtainthe desirable results of the invention, the molten metal solvent used informing the silicon/metal solution is comprised of a mixture of at leasttwo metals. The two metals include a first metal which, when pure, formsa low temperature eutectic with silicon at a first eutectic temperaturewhich is below the melting point of the metal. The molten metal solventalso includes a second metal which has a melting point below the firsteutectic temperature. In accordance with a preferred embodiment, thefirst metal is gold and the second metal is tin, and the gold and tinare mixed in the molten metal solvent in a ratio by weight in the rangeof from about 1:4 to about 3:7 of gold:tin.

In particular the invention provides a method of coating a materialsurface with a thin film of silicon comprising the steps of dissolvingsufficient silicon in a molten metal solvent at a first temperature suchthat the percentage of silicon is greater than the saturated percentagelevel at a second temperature, bringing the silicon/metal solution intocontact with an area of the material surface, and controllably varyingthe temperature of the silicon/metal solution from the first towards thesecond temperature at a rate corresponding to a desired rate ofdeposition of silicon, and wherein the molten metal solvent comprises amixture, or an alloy, of: a first metal which, when pure, forms a lowtemperature eutectic with silicon at a first eutectic temperature whichis below the melting point of the metal, and at least one second metalwhich has, in the pure state, a melting point below the first eutectictemperature or which, if forming an alloy with the first metal, forms aeutectic with said first metal at a eutectic temperature below the firsteutectic temperature.

The invention further provides a method of coating a material surfacewith thin film silicon by dissolving silicon in a molten metal solution,bringing the silicon/metal solution into contact with an area of thematerial surface, and by controllably varying the temperature of thesilicon/metal solution at different rates about the area so as to obtaindifferent desired rates of deposition of silicon about the area.

An important aspect of the invention is a solution growth method whichis used to deposit a thin film semiconductor coating on a supportingsubstrate. The method comprises dissolving the semiconductor in a moltenmetallic solvent, subsequently precipitating the semiconductor from thesolution comprised of the metal solvent and dissolved semiconductor intoa supporting substrate. The method is characterized in that the solventis a mixture of a first and a second metal. The first metal forms aeutectic with the semiconductor having a eutectic temperaturesubstantially below the melting point of the first metal when the firstmetal is substantially pure. The second metal or mixture of secondmetals which, in their substantially pure state, have a melting pointbelow the eutectic temperature of the first metal and the semiconductorand the second metal or mixture of second metals being substantiallycompletely mixable with the first metal. The first metal is preferablygold and the semiconductor is preferably silicon.

A primary object of the present invention is to provide a method ofdepositing a thin layer of silicon onto the surface of a material bycontrolling the temperature of a silicon/metal solution so as tocontrollably regulate the supersaturation of the silicon within thesolution and thereby control the evolution silicon from the solutiononto the material surface and form the thin silicon film.

Another object of the invention is to provide a silicon/metal solution,which solution is comprised of silicon and at least two metals, whichmetals are preferably gold and tin, and/or functional equivalentsthereof with respect to the methodology of the present invention. Forexample, the method can comprise the steps of dissolving silicon in asolution of gold and tin wherein the gold and tin are present in a ratioof from 7:3 to 4:1 (gold:tin by weight) to form a solution of silicon,gold and tin, the dissolving being carried out at a first temperaturebelow about 600° C. until the silicon reaches approximately thesaturation level of silicon in the solution; wetting a surface of theglass sheet by contacting the solution with the sheet and maintainingthe surface in contact with the solution; precipitating silicon from thesolution onto the surface by lowering the temperature of the solution;and controlling the rate of growth and final thickness of the siliconfilm by controlling the rate of change of the solution temperature andthe length of time the surface remains in contact with the solution.

An advantage of the present invention is that the processing can becarried out in an efficient and economical manner.

Another advantage of the present invention is that the thin silicon filmformed using the method of the invention is highly pure and uniform withrespect to its composition, thickness, and surface smoothness.

A feature of the present invention is that the method can be carried outusing relatively low temperatures.

Another object of the invention is to provide silicon coated materialswherein the silicon coating is provided on the material by using methodsof the invention to thereby obtain a highly pure and uniform siliconcoating on the material.

Another object of the invention is to provide soda lime glass sheetsurfaces which have silicon coated thereon wherein the silicon coatingis provided by a method of the invention.

Yet another object of the invention is to provide for solar cells whichsolar cells include soda lime glass sheets having silicon coated thereonwherein the silicon coating is provided by a method of the invention.

An advantage of the present invention is that thin film silicon solarcells can be efficiently and economically produced using the siliconcoating methods of the invention.

These and other objects, advantages and features of thesilicon-deposition method of the invention will become apparent to thosepersons skilled in the art upon reading the details of the compositions,steps, processing and usage as more fully set forth below, referencebeing made to the accompanying drawings forming a part hereof.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects,advantages and features will become apparent to those skilled in the artby reference to the accompanying drawings as follows:

FIG. 1 illustrates a continuous multiple cell formation technique of theinvention.

FIG. 2 illustrates a discrete multiple cell formation technique of theinvention.

FIG. 3 is a cross-section of a solar cell device of the invention havingmultiple parallel solar cell strips which overlap.

FIG. 4 illustrates a continuous multiple layer, multiple cell formationtechnique of the invention.

FIG. 5 is a cross-section of a solar cell which is a variation of thedevice in FIG. 3, having normally-doped regions and heavily dopedregions.

FIG. 6 illustrates a method of the invention for forming semiconductorstrips having variations in doping concentration.

FIG. 7 is a cross-section of a solar cell device having stacked solarcells connected in series.

FIG. 8 is a cross-section of a solar cell device having cells protectedby a bypass diode.

FIG. 9 is a cross-section of a solar cell device having double junctionconnections.

FIG. 10 is a cross section of a solar cell device having multiplestacked solar cells connected in parallel.

FIG. 11a is a cross-section of a solar cell device illustrating howdeposited layers using the method of the invention can be used toincorporate regulating circuitry as an integral part of the solar celldevice. The portion of the device depicted shows a Zener diode connectedto an npn transistor, connected to a solar cell.

FIG. 11b is a schematic for a simple voltage regulator for a solar panelwith (X+Y) series connected cells. X and Y can be varied as desired.This circuit is the representation of the deposited layers shown in FIG.11a, with additional contacts to the transistor and to the Zener diode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the present method of depositing silicon films on a material isdescribed, it is to be understood that this invention is not limited tothe particular process steps or metals described as such steps andmetals may, of course, vary. It is also to be understood that theterminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting, since the scope ofthe present invention will be limited only by the appended claims.

It must be noted that as used in this specification and the appendedclaims, the singular forms "a" "an" and "the" include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to "a metal" includes mixtures of metals of the type beingdescribed and reference to "the silicon deposition step" includes one ormore such steps of the general type being described, and so forth.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly used and understood by one of ordinaryskill in the art of silicon films, silicon deposition and solar cells.Although variations on the methods and materials similar to orequivalent to those described herein may be used to practice the methodsof the invention and/or obtain the products produced by the methods ofthe invention, preferred methods and materials known to the applicantsare described herein. Publications mentioned in this disclosure areincorporated herein by reference in order to disclose the specificsubject matter referred to in connection with the publication.

In accordance with the present invention, it is necessary to choose atleast one metal which is referred to a metal of low melting point and atleast one metal which forms a eutectic with a suitable semiconductivematerial such as gold at a low eutectic temperature. Although theinventors have found that certain metals are clearly useful andpreferred in connection with the present invention, the general conceptof the invention is not limited by the particular metals disclosedherein.

In general, the method of the invention involves dissolving silicon ineither an alloy or in a mixture of melted metals to form a solution andthen regulating the temperature of the solution in order to cause thesilicon in the solution to precipitate out of solution and deposit on asubstrate to be coated. Applicants have determined that by choosingparticular metals based on their physical characteristics (meltingpoint, solubility, etc.), it is possible to obtain an improved method bywhich silicon films can be efficiently and economically deposited on agiven substrate surface. Once the solution is formed by dissolving thesilicon in the metals, the silicon is generally deposited out bylowering the temperature of the solution so that the concentration ofsilicon within the solution will exceed its saturation point (becomesuper saturated) and precipitate out of solution. However, it ispossible to cause the silicon to precipitate out of solution by raisingthe temperature when the solution is brought into contact with a sourceof additional metal, such as tin, which additional metal (at the highertemperature) will enter the solution and force silicon to beprecipitated out of solution. Based on these concepts, it can be seenthat the method of the invention can be carried out in a variety ofdifferent ways using a variety of different metals and proportionalamounts of such metals. Some specific examples and preferred embodimentsare given below.

Examples of suitable metals with a low melting point include Bi, Cd, Ga,Hg, In, Pb, Sn, Tl and Zn. Examples of metals which form a eutectic withgold with a low eutectic temperature include Al, Bi, Cd, Ga, In, Pb, Sb,Sn and Tl and mixtures such as low temperature eutectics containing theabove metals or metals completely miscible with gold such as Ag, Cu, Ni,Pd and Pt. Other metals may be added to the molten metal solvent forother purposes such as doping the silicon and examples of such metalsinclude Al, As, Ga or Sb. The combination of Ag with Sn and/or Bi areparticularly preferred as metal solvents to which silicon is added.

The advantage of mixing such metals with gold is that the solubility ofsilicon in these metals or mixtures of metals is invariably lower thanthat of silicon in gold at temperatures of interest (provided suchtemperatures are above the gold-silicon eutectic temperature). Whenthese metals of initial mixtures are mixed with gold, the solubility ofsilicon in the final mixture at the temperatures of interest will almostinvariably be bound at the higher extreme by its solubility in gold atthe lower extreme by its solubility in the initial mixture. In practice,it is found that the solubility in gold can be greatly reduced in thismanner. For example, the solubility (in atomic percent) we have measuredin gold-tin mixtures at 410° C. decreases from approximately 20% forpure gold to approximately 5.3%, 0.5%, and 0.2% as 20%, 25% and 30% oftin by weight is added to gold.

The advantage of this reduced solubility is that growth rates of thesilicons being deposited from the silicon/metal solutions are easier tocontrol resulting in better film quality and reduced inclusions. Afurther advantage is that it is possible to extend growth temperaturesto lower temperatures than possible with gold alone. For example, amixture of 20% tin by weight with gold remains molten to temperatures aslow as 270° C., appreciably lower than the gold eutectic of 363° C.These temperatures represent the lower bounds on the growth temperaturesfrom the respective solutions.

The silicon films deposited in this way could be used, inter alia, asthe active layers of solar cells or as the substrates for transistorfabrication for a liquid crystal display television screen. Other useswill undoubtedly be available and will become apparent to those personsskilled in the art upon reading this disclosure.

Metals such as tin that have a strong effect upon the solubility ofsilicon in gold allow a variation upon the method of growth ofsemiconductor films from molten metal alloy solutions. The addition oftin to a solution of silicon in gold greatly decreases the solubility ofthe silicon in the solution. If originally saturated with silicon,silicon would precipitate out due to the addition of tin. If a solutionof gold, silicon and tin were saturated with both silicon and tin andplaced in contact with a tin rich compound such as pure Sn or AuSn ortin rich alloys of such materials, heating would cause more tin todissolve in the solution. The increased concentration of tin in thesolution would decrease the solubility of silicon causing silicon toprecipitate out. Hence, solution growth could be obtained in thisparticular embodiment by heating the solution rather than by theestablished approach of cooling it. This could have numerous advantagessuch as in the control of convection currents and of impurity andinclusion incorporation into the film.

Experiments have shown that while pure bismuth and lead have virtuallyno ability to dissolve silicon, gold added in the region of 5-50% byweight to either of these metals provides an alloy with good control ofsilicon deposition at below 400° C. and with the lower gold content thealloys can be produced relatively cheaply.

The method can be extended to other molten metal alloy solvents and toother dissolved semiconductors for example Sb/Au alloys for the moltenalloy and group III-V compounds and their alloys as the semiconductor.

The suitability of molten metal solvents for use in connection with amethod of the invention can be easily determined. First it is necessaryto document the melting and eutectic point temperatures. If thesetemperatures are favorable, experimentation is carried out regarding theability of the solvent to dissolve the desired group III-V compound oralloy. It is preferred that the solubility of the group III-V compoundor alloy in the metal solvent is low in the desired depositiontemperature range, say 0.1% to 5% (atomic).

In one embodiment the above methods will include the step ofincorporating Ge into the solution such that the material deposited ontothe supporting layer will be an alloy of Si and Ge.

By incorporating Ge into the solutions so that it is incorporated intothe deposited film additional advantages arise. The addition of Ge tendsto reduce the deposition temperature as well as the bandgap of theSi_(x) Ge_(1-x) alloy which will be deposited. As taught by Green (M. A.Green, IEEE Trans. Electron Devices, Vol. ED-31, pp. 681-689, 1984,incorporated herein by reference), recombination at cell surfacesbecomes more important than recombination in bulk regions as cellthickness decreases. By incorporating more Ge into the bulk regions orpart of the bulk region of thin cells than at surfaces, a balancebetween surface and bulk recombination can be maintained. This will givea performance advantage arising from increased light absorption in thereduced bandwidth bulk regions.

In some embodiments of the invention, an optically transparent seedinglayer is used between the supporting layer and the deposited silicon (orSi_(x) GE_(1-x)) film. Throughout the specification and claims, x is anynumber such that the atom or atoms it is associated with become electroncompatible with and allow the formation of bonds with the other atom oratoms in the molecule. The purpose of this layer is to establish a goodcrystalline structure onto which the silicon can be deposited with asimilarly good crystalline structure. This removes the dependency onusing crystalline quality silicon for the supporting substrate layer.The seeding layer will typically have a relatively poor crystallinestructure at the interface with the supporting layer which will improveas the layer grows away from the supporting layer. By making the layertransparent, the distance required to establish good crystallinitybecomes non-critical. Preferred choices for the seeding layer arematerials such as ZnS, CaF₂, GaP, A/P and BP onto which the epitaxialgrowth of silicon is possible. These can be deposited by any one of avariety of known techniques although deposition from metallic solutiongives the highest level of consistency with the subsequent deposition ofthe silicon layer. This layer could also serve to passivate the surfaceof the silicon film immediately in contact.

In a preferred embodiment of the invention a self-supporting glass sheetis used as a supporting superstrate for the deposition of silicon film.The lower the deposition temperature, the wider the range of glass typewhich can be used. For example, at high temperatures fused silica orvicor could be used. Borosilicate glass would be suitable at lowertemperatures. At deposition temperatures below 600° C., less expensivesoda lime glass would be used. In present solar cell modules based onself-supporting silicon wafers, low-iron soda lime plate glass of about3 mm thickness generally is used as the structural layer within themodule. Lowest cost would occur if the supporting superstrate of thepresent invention deviated as little as possible from this baseline.

Nucleation of crystal growth centers on the glass superstrate could beencouraged by the formation of structure on the side of the superstrateonto which the film or its seeding layer is deposited. Structure couldbe formed by mechanical processes such as rolling or by chemical orplasma etching. This structure could then encourage preferredorientations in the crystal film. It would also affect the direction oflight entering the silicon film through the superstrate. The formationof structure on the superstrate could be used to help trap weaklyabsorbed light into the silicon film. An example of such a desirablegeometrical structure which could be formed in the superstrate toenhance such effects would be small pyramids, preferably impressed intothe superstrate, tilted with respect to the original superstratesurface. The less controlled structure which results when glass ischemically etched would also be suitable. Any other method whichproduces a high density of small pores in the surface of the surfaceonto which the films are to be deposited would also be suitable, such asplasma etching.

According to further aspects of the invention, all the cells of theentire module are formed simultaneously on the supporting substrate.Several new techniques have been developed to implement the simultaneousformation of multiple cells. One is the portioned solution growth methodshown in FIG. 1. In this case the molten solution 1 containing thedissolved silicon is held in a suitable container 2 divided intocompartments 3 by thin partitions 4. These portions 4 may be fixed withrespect to the container 2 or may be able to slide with respect to it toprovide more intimate contact with the layer (the workpiece 5) ontowhich the solution film 6 is to be deposited. The purpose of thepartition 4 is to prevent the solution from wetting the workpiece 5 inthe region immediately underneath or adjacent to the partition 4. Bycooling the molten solution 1, or by arranging for a temperaturegradient from its upper levels held in intimate contact with a source ofsilicon (and possibly Ge) to the workpiece 5, silicon will be deposited.If the workpiece 5 is held stationary, the deposition will occur in theregions of the workpiece 5 under the separate chambers 3. The depositedlayers 6 will reflect the geometry of the chambers 3 in this case. Ifthe workpiece is moving slowly with respect to the solution, stripes 6of deposited silicon films (as shown in FIG. 1) can be formed.

Although the partitions 4 represent a mechanical solution as describedabove to prevent deposition in selected areas, thermal or electricalsolutions are also possible which can achieve the same purpose. Forinstance, by holding the partitions 4 at higher temperature than thesolution, any solution under the partition 4 would be hotter than thesurrounding solution 1 and hence supersaturation with silicon in theseregions could be prevented. This would prevent deposition in theseregions.

If the workpiece 5 is moving with respect to the solution 1, a pointsource of heat such as provided by a heated needle could be used to givethe strip structure to the deposited layer 6 similar to the final resultof FIG. 1. The heated solution 1 in the vicinity of the needle could beused to dissolve a small region in the deposited film as it passesunderneath. If the workpiece 5 is transparent, a laser beam illuminatingthe solution from underneath the workpiece 5 could provide a convenientsource of such localized heat for this purpose. If a strip laser wereused, deposition could be prevented along the strip illuminated by thelaser as in the case of the heated partition.

The thickness of the deposited layers 6 can be controlled by thetemperature of the solution 1 including gradients within it as well asthe length of time the solution 1 stays in contact with the workpiece 5.The doping level in the deposited layer 6 can be controlled bydepositing the silicon from a metal solution where the solvent 1 acts asa dopant in silicon, or by dissolving dopants in the solution, or acombination of the above. As shown later, successive layers of differentdopant level or type can be built up aligned or offset with respect toeach other to give desirable device structure. If the workpiece 5 isstationary, a great deal of flexibility is available as to the spatialdistribution of these layers. If the workpiece 5 is moving, as in FIG.1, additional techniques ar necessary to allow variations in depositedfilm properties in the direction of the workpiece motion. A techniquefor achieving this would be by cycling the temperatures of the metallicsolutions 1. When the temperature is highest near the substrate, thesolution 1 will no longer be supersaturated with silicon and depositioncan be made to stop. When coolest, deposition will be strongest.Alternatively, localized heating or cooling of substrate 5 provides thesame effect with perhaps more ease due to the lower thermal mass of thesubstrate 5 relative to the metallic solutions 1. By either means,patterned structure in the direction of motion can be obtained asillustrated in FIG. 2.

As indicated above, one preferred embodiment of the invention uses acombination of gold with tin and/or bismuth as the solvent fordissolving silicon. The solution formed is then used in a preferredmethod of the invention which is carried out by maintaining atemperature gradient, or more basically a composition gradient,throughout the solution during the deposition process step. There are anumber of means for maintaining the temperature gradient includinglocalized, direct heating. In accordance with a preferred embodiment ofthe invention, the step of controllably varying the temperature of thesolution involves varying the average temperature of the solution whilemaintaining a temperature gradient across the contact area between thesolution and the material surface where the silicon is to be depositedso as to affect a desired rate of deposition of silicon which is gratedacross the contact area between the solution and the material surface.

The role of temperature gradients from top to bottom of the moltensolution has already been mentioned. Lateral temperature gradientswithin the solution can also be used to give patterned structure as inFIG. 2.

Using the techniques as described, solar cell structures can be built upusing overlying strips 6 of deposited silicon of different dopantconcentration, thickness and Ge content. An advantage of the presentinvention's ability t overlap and offset layers relative to each otheris that structures such as those described can be formed withoutrequiring separate masking, photolithography or additional processingsteps.

FIG. 3 shows one such implementation. The structure in the directioninto the page remains the same so the different layers shown take theform of long strips 6. A schematic of the process by which the layerscould be deposited is shown in FIG. 4. Shown in FIG. 3 are several solarcells each with an n+-p-p+ junction structure as well known in the art.The cells, however, are connected together in a series connection at theareas 8 where contact is made between the n+ and p+ region. Contactsbetween such heavily doped regions, instead of acting as a rectifyingjunction, act as a low resistance "ohmic" contact. Alternatively,rectifying qualities between the n+ and p+ regions could be destroyed bydeliberately damaging the crystallographic quality of those regions.Also incorporated into the cells is an "isolation region" 8a whereby thecontact region 8 is isolated from the main body of the cell by the highlateral resistance of the p-type region. The n+ and p+ regions need tobe sufficiently highly doped and sufficiently thick to present a smalllateral resistance to current flow. This is most challenging for the n+region since there is a conflicting constraint on its thickness. This isimposed by the fact that it must be sufficiently thin for carriersgenerated by light near its interface with the superstrate or seedinglayer to find their way to its junction before recombining. One approachto reducing the losses involved in this tradeoff is to arrange for alateral temperature gradient within the molten bath 1 to give a lateralthickness variation or a lateral doping level variation or a combinationof the above. The geometry of the bath 1 could also be used to controlthickness as subsequently described. The p+ region could be similarlygraded as shown, although the benefits would be smaller in this case.However, there exists other benefits through introducing lateralthickness variations such as by facilitating control of the reargeometry of the cell to allow optimization of light trapping within thecell.

An alternative or complementary approach would be the use of"semiconductor fingers". A method of forming these, together with theresulting structure is shown in FIG. 6. By periodically cycling thetemperature of the first bath 3b, highly doped n++ finger regions couldbe periodically deposited of width similar to the dimensions of the bathfrom which the solutions are deposited. These would serve to reduce thelateral resistance of the n+ layer of the cell while restricting thedeleterious effects of excess doping and thickness to a fraction of thecell area. Contact between successive cells could be restricted to then++ areas. This could reduce the area required for the isolation regionin some implementations. Although the n++ regions are shown as sharplydefined units in FIG. 6, the temperature variation in the correspondingbath or substrate could be controlled to smear this region out. In fact,a transition from n++ to n+ properties could be achieved by temperaturecontrol in a single bath, eliminating the need for the second bath 3ashown in FIG. 6. The nonrectangular shape of the first bath will promotea thickness variation in the final film 6. It should be noted that thestructures described have the potential for not requiring metalcontacts. This feature has numerous advantages. These include betterdurability by eliminating the potential for metal/silicon interactions,the elimination of losses due to shading, and the simplification ofprocessing. However, metal contacts could also be included by depositiononto the supporting layer or deposited film by standard techniques.Solution growth of metal silicide contacts with an epitaxialrelationship to silicon would be a particularly attractive option.

Multiple cells could be stacked on top of each other and seriesconnected as shown in FIG. 7. In this case, the germanium content in thelightly doped regions could increase in each successive cell to takeadvantage of the increasing red content of the light as it passesthrough the cells. One advantage of such a tandem arrangement of cellsis that it reduces the lateral current flow in each of the heavily dopedregions of the cell. Another improvement could be the incorporation of abypass diode 11 across each cell as shown in FIG. 8.

If the cell protected by the bypass diode 11 is generating less than itsdesigned current output either due to partial shading of the module inthe filed or manufacturing defects, the bypass diode 11 becomes forwardbiased and provides a current path around the cell. This not onlyimproves the field reliability of the module, it also improvesmanufacturing yield. By incorporation of a few extra cells in themodule, modules without all cells fully operational would still meetdesign specifications.

An alternative approach to building up cell structures would be thedouble junction approach of FIG. 9. In this case, the p-type layer wouldbe more highly doped than for the structure of FIG. 3. This region formsa junction with the n+ regions immediately above and below it, withthese junctions connected in parallel. If carrier lifetimes in thep-region are large so that diffusion lengths are larger than thethickness of this region, the generated current will partition itselfbetween these two junctions in a way which will tend to take possibleresistance losses along the top n+ layer into account. If diffusionlengths are shorter than this, a thin heavily doped p+ core couldadvantageously be used within the p-region to reduce its lateralresistance. The rear n+ region is deposited in two steps in thestructure illustrated in FIG. 9 to allow the highest level of control ofalignment to the p+ region providing the interconnection betweensuccessive cells.

Again, multiple cells can be stacked on each other as shown in FIG. 10.In this case, all the cells in the stack are connected in parallel. Theadvantage of this arrangement is that it makes it easier for thediffusion lengths to be larger than the p-type region thicknesses. Italso means that each layer has to carry less current, reducing lateralresistance loss. A by-pass diode can also be incorporated into the rearof the cell in a manner similar to that previously described.

The flexibility of the described invention to produce doped or undopedsemi-conductor layers in any desired position relative to one anotherfacilitates the concept of including additional circuitry as an integralpart of the panel (module) itself. Configurations of diodes 11,transistors 9 and resistors 10, such as in FIGS. 11a and 11b, can beincorporated into the structures using the described techniques andmethods providing the potential to regulate the output from each modulein a manner whereby the output was completely temperature independentfor the range of operating temperatures likely to be experienced by themodule in the field. FIG. 11a shows such a device having Zener diode 11connected to npn transistor 9 by a thin p-type region which is thenconnected to the first solar cell. FIG. 11b shows X and Y seriesconnected cells 12 connected to a simple voltage regulator, and is thecircuit representation of the structure of FIG. 11a. In conjunction withthe bypass diodes 11 (FIG. 8) it would then be feasible to have a modulewith an output (I-V curve) that would be independent of temperaturechanges, failures of small numbers of cell and/or shading of regions ofthe module.

It will be appreciated that the use of the present invention toincorporate additional circuitry as an integral part of the module,although being described in terms of regulating the module output, maybe used to form any number of other desired effects or results.

Note that in all previously described structures, all n-type regionsmentioned could be replaced by p-type regions provided p-type regionswere simultaneously replaced by n-type. Note also that although theinvention has been described specifically in terms of silicon layersincluding alloys with Ge, the invention is also applicable to othersemiconductors and alloys. Similarly, although the specified method forgrowth of doped semi-conductor layers was by precipitation from moltensolution, it will be appreciated that many of the discussed techniques,features and structures are equally applicable to other methods of filmgrowth or formation.

The approaches described in the text accompanying FIGS. 1 to 11 couldalso be applied to solutions where the silicon is dissolved in moltenmetals and metal alloys other than those based on gold. For example,solutions based on Al, As, Bi, Cd, Cu, Ga, Hg, In, Ni, Pb, Pd, Pt, Sb,Sn, Tl and Zn or their alloys would also be suitable.

The approaches described in the text accompanying FIGS. 1 to 11 couldalso be modified in a way which will become apparent to those skilled inthe art who have read this disclosure to allow the deposition of thepatterns required for other applications such as large area displays.Since costs are less critical in this application, a variety of knowntechniques such as photolithography could be used in conjunction withlow temperature deposition from the novel gold solutions to effect thedesired patterns.

We claim:
 1. A solution growth method for depositing a thin filmsemiconductor coating on a supporting substrate, the method comprisingdissolving said semiconductor in a molten metal solvent to form asolution of said metal solvent and dissolved semiconductor, subsequentlyprecipitating said semiconductor from the solution onto a supportingsubstrate, characterized in that said solvent is a mixture of:a firstmetal which forms a eutectic with said semiconductor having a eutectictemperature below the melting point of said first metal whensubstantially pure; and a second metal or mixture of second metalswhich, in their substantially pure state, have a melting point below theeutectic temperature of said first metal and said semiconductor and saidsecond metal or mixture of second metals being substantially completelymiscible with said first metal.
 2. A method as defined in claim 1wherein said first metal is gold.
 3. The method as defined in claim 2,wherein the semiconductor is silicon.
 4. The method as defined in claim1 wherein said step of precipitating the semiconductor from the solutioncomprises controllably changing the temperature of the solution from afirst temperature to a second temperature and wherein the solubility ofthe semiconductor in the solution at the second temperature is lowerthan the solubility of the semiconductor in the solution at the firsttemperature.
 5. The method as defined in claim 4 wherein the secondtemperature is lower than the first temperature.
 6. A method as definedin claim 2 wherein said step of precipitating the semiconductor from thesolution comprises controllably changing the temperature of the solutionfrom a first temperature to a second temperature and wherein thesolubility of the semiconductor in the solution at the secondtemperature is lower than the solubility of the semiconductor in thesolution at the first temperature.
 7. The method as defined in claim 6wherein the second temperature is lower than the first temperature. 8.The method as defined in claim 6 wherein the second temperature ishigher than the first temperature and wherein a source of said secondmetal or mixture of second metals is maintained in contact with thesolution at least during the step of precipitating said semiconductorfrom the solution so as to maintain said solution substantiallysaturated with said second metal or mixture of second metals.
 9. Themethod as defined in claim 1 wherein, at least during precipitation ofthe semiconductor, the temperature of the solution is maintainedsubstantially constant relative to time and is maintained at atemperature gradient through the solution ranging from a firsttemperature to a second temperature wherein at the second temperaturethe solubility of the semiconductor in the solution is lower than thesolubility of the semiconductor in the solution at the first temperatureand wherein the temperature of the solution at an area in contact withsaid substrate is substantially at said second temperature.
 10. Themethod as defined in claim 9 wherein the second metal or mixture ofsecond metals is at least one metal selected form the group consistingof: Bi, Cd, Ga, Hg, In, Pb, Sn, Tl, Zn, Ag, Cu, Ni, Pd and Pt.
 11. Themethod as defined in claim 1 wherein the second metal or mixture ofsecond metals is at least one metal selected form the group consistingof: Al, Bi, Cd, Ga, In, Pb, Sb, Sn and Tl.
 12. The method of claim 4wherein the support substrate is soda lime glass.
 13. The method ofclaim 9 wherein the support substrate is soda lime glass.
 14. A methodof coating a glass sheet with thin film silicon comprising the stepsof:dissolving silicon in a solution of gold and tin wherein the gold andtin are present in a ratio of from 7:3 to 4:1 (gold:tin by weight toform a solution of silicon, gold and tin, the dissolving being carriedout at a first temperature below about 600° C. until the silicon reachesapproximately the saturation level of silicon in the solution; wetting asurface of the glass sheet by contacting the solution with the sheet andmaintaining the surface in contact with the solution; precipitatingsilicon from the solution onto the surface by lowering the temperatureof the solution; and controlling the rate of growth and final thicknessof the silicon film by controlling the rate of change of the solutiontemperature and the length of time the surface remains in contact withthe solution.
 15. A solution growth method for depositing a thin filmsemiconductor coating on a supporting substrate, the methodcomprisingdissolving said semiconductor in a molten metal solvent toform a solution of said metal solvent and dissolved semiconductor andsubsequently precipitating said semiconductor from the solution onto asupporting substrate, characterized in that said solvent in a mixtureof: a first metal which forms a eutectic with said semiconductor, thefirst metal being selected from the group consisting of Au, Al, As, Bi,Cd, Cu, Ga, Hg, In, Ni, Pb, Pd, Sb, Sn, Tl and Zn; and a second metal ormixture of second metals which, in their substantially pure state, havea melting point below the eutectic temperature of said first metal andsaid semiconductor and said second metal or mixture of second metalsbeing substantially completely miscible with said first metal.
 16. Amethod as defined in claim 15 wherein said first metal is gold.
 17. Themethod as defined in claim 14, wherein the semiconductor is silicon. 18.The method as defined in claim 15, wherein said step of precipitatingthe semiconductor from the solution comprises controllably changing thetemperature of the solution from a first temperature to a secondtemperature and wherein the solubility of the semiconductor in thesolution at the second temperature is lower than the solubility of thesemiconductor in the solution at the first temperature.
 19. The methodas defined in claim 18 wherein the second temperature is lower than thefirst temperature.
 20. A method as defined in claim 16 wherein said stepof precipitating the semiconductor from the solution comprisescontrollably changing the temperature of the solution from a firsttemperature to a second temperature and wherein the solubility of thesemiconductor in the solution at the second temperature is lower thanthe solubility of the semiconductor in the solution at the firsttemperature.
 21. The method as defined in claim 20 wherein the secondtemperature is lower than the first temperature.
 22. The method asdefined in claim 20 wherein the second temperature is higher than thefirst temperature and wherein a source of said second metal or mixtureof second metals is maintained in contact with the solution at leastduring the step of precipitating said semiconductor form the solution soas to maintain said solution substantially saturated with said secondmetal or mixture of second metals.
 23. The method as defined in claim 15wherein, at least during precipitation of the semiconductor, thetemperature of the solution is maintained substantially constantrelative to time and is maintained at a temperature gradient through thesolution ranging from a first temperature to a second temperaturewherein at the second temperature the solubility of the semiconductor inthe solution is lower than the solubility of the semiconductor in thesolution at the first temperature and wherein the temperature of thesolution at an area in contact with said substrate is substantially atsaid second temperature.
 24. The method as defined in claim 23 whereinthe second metal or mixture of second metals is at least one metalselected form the group consisting of: Bi, Cd, Ga, Hg, In, Pb, Sn, Tl,Zn, Ag, Cu, Ni, Pd and Pt.
 25. The method as defined in claim 15 whereinthe second metal or mixture of second metals is at least one metalselected form the group consisting of: Al, Bi, Cd, Ga, In, Pb, Sb, Snand Tl.
 26. The method of claim 18 wherein the support substrate is sodalime glass.
 27. The method of claim 23 wherein the support substrate issoda lime glass.